The present invention relates to an apparatus for reproducing information, and more particularly to a signal processing system for reading information recorded on the basis of the sampled servo system onto an optical disk, or the like.
As media for recording various digital data, music information, image information, or the like, optical disks are known. Since writable optical disks can be subjected to a write once operation, in order to record information by the user, the demand therefor is increasing. These write once read optical disks are called a WORM (Write Once Read Multiple) optical disk or a DRAW (Direct Read After Write) optical disk. The write once read disks are such that information can be written thereonto only once. In this case, writing is carried out by converging a laser beam onto a recording surface to transform the laser beam to a thermal energy at the convergent point (of a very small area) to perforate the recording film by fusing action, etc., due to heat generated, to form signal pits, and thus write information thereinto. For the recording format, the sampled servo system is used. At the time of writing information, the information is modulated in accordance with 4/15 modulation (.sub.15 C.sub.4 : 4 out of 15 code) method. Playback or reproduction of the information thus recorded is carried out by irradiating a laser beam onto a recording track on a recording surface, including signal pits formed in a manner stated above, to apply photoelectric conversion to a reflected laser beam by using a photodetector to apply analog to digital (A/D) conversion to an electric RF signal (RF: Radio Frequency) to demodulate that digital signal by using a 4/15 decoder.
In the conventional apparatus for reproducing information of the sampled servo system, the problem therewith is that resolution of the A/D converter is not sufficiently exhibited, resulting in a poor utilization efficiency.
Namely, information subjected to 4/15 modulation by the sampled servo system is changed to that in a digital form by means of the A/D converter, and is then demodulated by means of the 4/15 decoder. This demodulation is carried out by so called Differential Detection system to carry out sampling by servo fields preformatted at 1376 portions in one track to extract 4 bits of high order level by comparison between amplitudes of data field read signal at respective channel bits every sampling period.
The channel bit data necessary for demodulation is only 4 channel bits of high order level. Other bits of low order level are unnecessary. Accordingly, it is basically unnecessary to apply A/D conversion to such bits. In the case of the prior art, however, since high and low level reference signals are set so that the input range of the A/D converter is caused to be in correspondence with the full scale value of the data field read signal as an analog input, even unnecessary data is subjected to A/D conversion.
On the other hand, from a viewpoint of resolution, it is not necessary to apply A/D conversion to the servo field read signal with a higher precision than that of the data field read signal, but it is required for reading the servo fields to allow the input range of the A/D converter to be substantially in correspondence with the full scale of the analog input. In the prior art, however, since the servo field read signal for which high precision is not required and the data field read signal for which high precision is required are subjected to A/D conversion in the same range, an A/D converter of high resolution must be used. In this connection, the resolution of the A/D converter is generally determined by the number of bits. The higher the resolution is, the more the number of bits is. This disadvantageously leads to increased cost of the apparatus for reproducing information.